The present invention relates to a felt for paper manufacture and the method for producing it.
Recently, the operating speed of paper manufacturing machines has been increased remarkably, and with this increase of the operating speed, the load imposed on press parts utilized for squeezing a wet web by press rolls has been increased. When the operation is carried out at an increased paper-producing speed, the time during which a wet web is retained in the nip portion between press rollers becomes shortened and therefore, the water discharge speed from the wet web should be increased at the pressing step.
The above-mentioned increase of the load is necessary to cope with this increase of the water discharge speed. However, if a high load is applied by the press rolls, the hydraulic pressure is increased when water is squeezed out from the wet web, and displacement of fibers in the wet web, that is, so-called paper breaking, is readily caused. Furthermore, the felt is flattened in a short time and becomes harder and the water permeability and air permeability of the felt are degraded. This phenomenon is called "sagging", and if sagging of the felt is caused, since voids, or empty spaces of the felt, for receiving water squeezed out from the wet web in the nip of the press rolls are reduced, removal of water from the wet web cannot smoothly be performed.
Recently, there has been developed an apparatus called "broad-width nip press", in which a wet web gripped between two felts is passed through one roll and a belt partially pressed to the roll and a strong pressing force is applied from the belt side to squeeze out water from the wet web. The felt of this apparatus is introduced into the nip portion while having voids sufficient to completely receive water squeezed out from the wet web, the felt receives water, and after the felt has separated from the nip portion, water is removed from the felt by suction or the like. Also in this apparatus, if sagging of the felt is caused, the capacity for receiving water becomes insufficient. Accordingly, it is important that sagging should be prevented as completely as possible.
If the felt for paper manufacture is completely non-compressible, even though the capacity of voids is sufficient, paper breaking is often caused because a high pressure is applied to the wet web at the moment when the wet web is introduced into the nip portion between rolls. Ideally, a felt for paper manufacture should have a certain compressibility providing a maximum pressure to the wet web over a period of a certain time and a certain non-compressibility necessary for retaining a sufficient capacity of voids. Of course, it is required that the recovering property after removal of the load should be high.
A so-called needle felt has been proposed as the felt for paper manufacture satisfying these requirements. This needle felt comprises a non-compressible ground fabric and a lap bonded to the ground fabric by needling, and retention of a sufficient capacity of voids is mainly attained by the ground fabric and retention of a necessary compressibility is attained by the lap. When the thickness of the ground fabric is increased so that deformation is hardly caused, voids are sufficiently retained and the ability to receive water from the wet web in the roll nip portion is increased. Accordingly, trials have been made to provide felts comprising a ground fabric having a double combination weave or a multiple combination weave such as a triple combination weave. However, a multiple weave fabric such as a triple weave fabric is practically insufficient in that formaton of a uniform texture by weaving is very difficult and since the resistance to bending is very high, handing is very difficult at the post treatment after the weaving operation or at the step of setting the felt at a paper manufacturing machine.